Support C# nullable references

[CodingOctocat]

Now in C#8+, the project has NRT enabled by default: <Nullable>enable</Nullable>, should sqlite-net enable [NotNull] by default? Maybe it can be added:

• Sqlite.UseNullableTypesMetadata = true // global not null for properties
• [CanBeNull] attribute // not null property but can be null in database